A vacuum tube is a type of electronic device that is usually sealed in a vacuum box to control the flow of electrons. In early 20th century, almost all of the electronic devices were made of vacuum tubes. However, the vacuum tubes have the drawbacks of high cost, short lifetime, big volume and low performance, They were largely replaced by solid state devices during the 1960s and 1970s. Only instruments that need high performance such as audio amplifiers, microwave stove, satellite transponders or even some of the fighter aircraft applications are still using vacuum tubes. Please refer to FIG. 1, it shows the circuit diagram of the conventional vacuum tube “Triode”, which includes a grid 1, plate 3, emitter 2 and filament 5. This additional control grid 1 modulates the current that flows from emitter 2 to plate 3.
Early electronics centered around the vacuum tube used to amplify, switch, or modulate electrical signals. It has been many decades since the vacuum tubes have been replaced by solid-state devices such as the MOSFET and BJT and diodes.
The vacuum tubes are still used in niche applications such as premier sound systems and high-power radio base stations. The transition from the vacuum tube to the solid-state device was not driven by the superiority of the semiconductor as a carrier transport medium but by the ease of fabrication, low cost, low-power consumption, lightness, long lifetime, and ideal form factor for integrated circuits.
The vacuum device is more robust than solid-state devices in extreme environments involving high temperature and exposure to various radiations.
The critical tradeoff is that the vacuum tubes yield higher frequency/power output but consume more energy than the MOSFET.
The vacuum is intrinsically superior to the solid as carrier transport, medium since it allows ballistic transport while the carriers suffer from optical and acoustic phonon scattering in semiconductors. The velocity of electrons in vacuum is theoretically 3×1010 cm/s, but is limited to about 5×107 cm/s in semiconductors.